Re: [PATCH 1/3] memcg: limit the number of thresholds per-memcg

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Aug 07, 2013 at 04:37:27PM +0200, Michal Hocko wrote:
> On Wed 07-08-13 09:58:18, Tejun Heo wrote:
> > Hello,
> > 
> > On Wed, Aug 07, 2013 at 03:46:54PM +0200, Michal Hocko wrote:
> > > OK, I have obviously misunderstood your concern mentioned in the other
> > > email. Could you be more specific what is the DoS scenario which was
> > > your concern, then?
> > 
> > So, let's say the file is write-accessible to !priv user which is
> > under reasonable resource limits.  Normally this shouldn't affect priv
> > system tools which are monitoring the same event as it shouldn't be
> > able to deplete resources as long as the resource control mechanisms
> > are configured and functioning properly; however, the memory usage
> > event puts all event listeners into a single contiguous table which a
> > !priv user can easily expand to a size where the table can no longer
> > be enlarged and if a priv system tool or another user tries to
> > register event afterwards, it'll fail.  IOW, it creates a shared
> > resource which isn't properly provisioned and can be trivially filled
> > up making it an easy DoS target.
> 
> OK, got your point. You are right and I haven't considered the size of
> the table and the size restrictions of kmalloc. Thanks for pointing this
> out!
> ---
> From cde8a3333296eddd288780e78803610127401b6a Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.cz>
> Date: Wed, 7 Aug 2013 11:11:22 +0200
> Subject: [PATCH] memcg: limit the number of thresholds per-memcg
> 
> There is no limit for the maximum number of threshold events registered
> per memcg. It is even worse that all the events are stored in a
> per-memcg table which is enlarged when a new event is registered. This
> can lead to the following issue mentioned by Tejun:
> "
> So, let's say the file is write-accessible to !priv user which is
> under reasonable resource limits.  Normally this shouldn't affect priv
> system tools which are monitoring the same event as it shouldn't be
> able to deplete resources as long as the resource control mechanisms
> are configured and functioning properly; however, the memory usage
> event puts all event listeners into a single contiguous table which a
> !priv user can easily expand to a size where the table can no longer
> be enlarged and if a priv system tool or another user tries to
> register event afterwards, it'll fail.  IOW, it creates a shared
> resource which isn't properly provisioned and can be trivially filled
> up making it an easy DoS target.
> "
> 
> Let's be more strict and cap the number of events that might be
> registered. MAX_THRESHOLD_EVENTS value is more or less random. The
> expectation is that it should be high enough to cover reasonable
> usecases while not too high to allow excessive resources consumption.
> 1024 events consume something like 16KB which shouldn't be a big deal
> and it should be good enough.

Is it correct that you fix one local DoS by introducing a new one?
With the page the !priv user can block root from registering a threshold.
Is it really the way we want to fix it?

-- 
 Kirill A. Shutemov

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